base for determining the features of road design that are most directly affected by the increase in volume and speed of traffic, features in which there has been the highest degree of obsolescence in the older highways. These roadway features relate to the alinement and profile, the plan of intersections, the clearances, and the horizontal dimensions of the highway cross section.
Many of the recent studies have been devoted to the dynamics of highway movement; and the results of these studies, combined with the evidence of traffic growth and distribution and other information resulting from the State highway planning surveys, form the basis of the geometric highway standards, previously described, which have been employed in rating the deficiencies of the interstate highway system, and estimating the cost of its adequate improvement.
WIDTHS OF SURFACES AND SHOULDERS
No features of a highway have a greater influence upon the safety and comfort of driving than the width and condition of the surface. On two-lane roads the width of pavement is most critical when vehicles meet, as it is then that drivers must allow sufficient clearance between vehicle bodies to assure complete safety, while at the same time maintaining a comfortable margin of safety from the pavement edge. The clearances necessary for safety at the speeds that vehicles are operated, added to the combined widths of the two vehicle bodies, constitute the needed pavement width.
Lane widths
Most of the freight-carrying vehicles now using public highways are 8 feet wide. The results of investigation of the transverse positions of vehicles in motion show that lanes 12 feet wide are the minimum that will provide ample clearances between commercial vehicles as they meet one another. This lane width will also be adequate in case the width of commercial vehicles is increased to 8.5 feet. It is only where the traffic streams are composed almost entirely of passenger cars that lane widths of 11 feet are satisfactory.
Roadside obstructions
These investigations also reveal that any objects such as retaining walls, bridge trusses, or headwalls adjacent to a roadway constitute a safety hazard and are an obstruction to the free movement of traffic, unless they are 6 feet removed from the normal pavement edge. If they are 3 or 4 feet away, however, their effect will not be critical enough to justify greater clearances from the normal pavement edge on long bridges and underpasses.
Shoulders
Shoulders capable of supporting all types of vehicles standing on them or passing onto them infrequently and in emergency at high speed during any weather conditions are essential for traffic safety. Adequate shoulders are also essential to realize the full capacity of the surface width. Without adequate shoulders, one disabled vehicle can reduce the capacity of both two-lane and multilane highways during peak periods by as much as 60 percent.
A shoulder width of 6 feet is required to insure that lateral obstructive objects will not decrease the effective pavement width; and an
